Definitely Naruto. He's the most likely candidate to have the Kyuubi doujutsu given his bond with the nine - tailed fox. Hinata could also play a role. Maybe she discovers a way to help Naruto control the doujutsu better, using her Byakugan to analyze the chakra flow related to it. Kakashi might be involved too, as he has experience with different types of doujutsu and could serve as a mentor to Naruto in understanding and using the Kyuubi doujutsu.
In 'naruto kyuubi doujutsu fanfiction', Naruto is a given main character. Another character could be Kurama, the Kyuubi itself. It could communicate with Naruto in a more direct way through the doujutsu, guiding him on how to use its power. Sakura might also be involved. She could use her medical ninjutsu knowledge to help Naruto if the doujutsu has any negative effects on his body. And Jiraiya could be part of the story too, as he was Naruto's mentor and might have known something about the potential of the Kyuubi doujutsu that he passed on to Naruto.
Naruto is usually a main character as he has a connection with the Kyuubi. Since he already has the Kyuubi sealed within him, it's logical that he would be the one to potentially unlock or develop the Kyuubi doujutsu. Sasuke might also be involved. His rivalry with Naruto could lead to interesting interactions in relation to the doujutsu, either trying to obtain it for himself or learning to counter it.
Naruto himself is often involved. His strong will and determination could make him either a target of hypnosis or someone who has to resist it. Sasuke is also a likely candidate, given his powerful doujutsu, the Sharingan. Hypnosis could potentially be used to manipulate his actions or unlock hidden aspects of his Sharingan powers.
There could be original characters as well. In fanfiction, authors often create new characters to add to the story. For example, a new ninja from a hidden village who has a special doujutsu that somehow connects with the Kyuubi's power. This character could become an ally or an enemy to Naruto. They could bring in new plot elements, such as a new form of chakra that is only accessible through the combination of their doujutsu and the Kyuubi's power, which would make for a very exciting and unique story.
One of the main characters could be Hinata. Since she has the Byakugan, a fanfiction might explore the idea of her evolving it into a godlike doujutsu. Another character could be Madara Uchiha. His return in some form and obtaining a godlike doujutsu would be an interesting plot point. And of course, there could be mysterious, unnamed characters that are the center of the story in the fanfiction, with their godlike doujutsu being the main focus.
Well, Naruto is the top one. As the Jinchuriki of the Kyuubi, he's at the center of it all. But don't forget about Jiraiya. He has a huge influence on Naruto and his past self could be a key factor in any time travel plot. And then there's Hinata. Her relationship with Naruto could be explored in new ways if time travel is involved. For example, she could help Naruto in the past in ways she didn't before, adding a new dimension to their relationship.
Obviously, Naruto is the main character as he is the one releasing the Kyuubi. Sasuke could also be involved, perhaps as an ally or an antagonist depending on the plot of the fanfiction. Sakura might be there too, trying to support Naruto in controlling the Kyuubi's power or dealing with the aftermath.
Naruto Uzumaki is often associated as he has ties to the Namikaze clan through his father. He could be a central character in exploring the Namikaze clan doujutsu, either as a potential wielder or someone trying to understand it. Another could be Minato Namikaze, Naruto's father. His own abilities and knowledge could be crucial in uncovering the secrets of the clan's doujutsu.
Sasuke often appears. Since he is Naruto's rival and friend, his relationship with Naruto might be affected by the Kyuubi's influence on Naruto. Kakashi could also be there, as Naruto's teacher, he might notice the changes in Naruto due to the Kyuubi's new role in his life.
Naruto himself is often a central character in 'naruto destruction fanfiction'. Since he is the main protagonist of the original series, fanfiction writers like to put him in situations where he has to deal with destruction. Sasuke is also frequently involved as his rival, and their relationship can be explored in the context of destruction, like if they have to work together to stop a greater threat of destruction.
Well, Naruto is obviously a main character in most of these. But there could also be Sasuke. He might be either an ally or an adversary in relation to the dagger. For example, if the dagger has a power that can either be used for good or evil, Sasuke could be tempted by its darker potential.
Some fanfictions might also include characters like Kakashi in important roles. As Naruto's former teacher, he could be a guiding figure or someone who has to adapt to Naruto's new level of power. He might also be involved in trying to keep Naruto's power in check if it starts to have negative effects on the ninja world.